For the past few days OS X is showing me updates for iMovie and it started downloading it. I changed my mind about iMovie and so deleted the application. But still it is trying to update iMovie and started downloading it again. How do I let AppStore know I don't need the update and don't show me future updates for iMove?

Try in your Purchased list - right click, Hide purchase. Never tried it, but it's a possibility. Some apps can be hidden from the updates list in the same way, but not all. [I can't test as mine doesn't need an update right now]
– TetsujinAug 11 '15 at 16:48

I had already deleted the App but AppCleaner didn't show any extra files to delete other than the app itself. I tried to bring up the option to cancel download. I saw the option and clicked on Cancel. The download was cancelled but it still shows in the available updates.!
– Mathew ParetAug 12 '15 at 0:46

@MathewParet. I've added some more options here, but I'm not sure what the problem is. The update list may just need to be reset. Have you performed a restart of the os?
– NiallAug 12 '15 at 1:08

I thought I will install update and the uninstall. But that would mean I have to waste about 2 G of bandwidth for nothing. So that's my last option. However let me try the app resetting thing. Will let you know.
– Mathew ParetAug 12 '15 at 3:08

You can select "Applications" from the menu bar on the AppCleaner's window and scroll down until you find the desired application.
You may select it(if it's locked/stock app, you may need to click twice faster in the checkbox). Going forward with the procedure by pressing the "Search" button will bring up a list of of all related files. Note that some of them might be locked as well, so the above double-click procedure will apply again.

Right after you open the app, you will have it as free space where you can Drag&Drop the application that you want to remove and it will find all related files that needs to be deleted

Note: However, this method may not apply for the stock Applications, so you may need to follow the other method.

Also note that at the end of both procedures you will be required to enter administrative password in order to give access to AppCleaner to move the application to the Trash.

I deleted the app with finder.
Then I still had the appstore prompting for download iMovie which was irritating so I ignored the prompt to update.
Then upon reading advice here I went back to finder/applications and there was a folder labeled imovie 9.09 so I sent that folder also to trash.
That solved my problem. Appstore is not prompting to update and imovie seems to be off my Mac now

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